The goal of this proposal is to advance the treatment of human colon cancer by the development of Fura+AZT-based drug combinations. Studies indicate that AZT-induced cytotoxicity and incorporation into DNA are related to intracellular a) pools of dttp, b) capacity to generate AZTTP and c) DTTP:AZTTP ratios. Proposed experiments will correlate the effect of Fura on these 3 parameters with its ability to increase AZT cytotoxicity in 2 human colon tumor cell lines (HCT-8 and GC3) with different sensitivities to AZT(+/-Fura) (Specific Aim 1, SA1). These studies will be extended in vivo to evaluate the effect(s) that Fura exerts on AZTTP synthesis and utilization in normal and human colon tumor tissue obtained from mice. An important aspect will be to correlate Fura-induced depletion of DTTP pools with its effect on [3H]- AZT incorporation into tissue nucleic acids (SA2). Methods to increase DTTP pools, reduce AZT(+/-Fura) cytotoxicity and increase its therapeutic index will also be developed. In vitro experiments will identify the relationship between cellular pools of dThd and Dthd nucleotides or the cytotoxicity of Fura+AZT, by their quantitation in cells incubated in varying concentrations of exogenous Dthd. The ability of the nucleoside transport inhibitor dipyridamole (DP), alone or combined with exogenous Dthd, to effect these parameters will also be assessed (SA3). In vivo, alterations in tissue Dthd pools, Dthd nucleotide pools and AZT(+/-Fura) incorporation into tissue nucleic acids produced by administration of exogenous Dthd, DP, or their combination, will be investigated (SA4). All of the above studies will serve as the basis for the development (in nude mice bearing human colon tumors) of various preclinical therapeutic regimens of Fura+AZT(SA5). Finally, through multi-center collaborations, these studies will a) serve to identify relevant parameters to be monitored in clinical trials and b) have immediate utility in the design of future clinical trials of Fura+AZT (SA6). In summary, it is anticipated that completion of these studies will have an immediate impact on the therapeutic options for the treatment of human colon cancer.